1. Technical Field
The present invention is relative to a nitride semiconductor light emitting device and fabrication method thereof, and to a nitride semiconductor light emitting device and a fabrication method thereof that can increase the optical power and enhance the reliability by increasing a hole carrier concentration contributing to the electric conductivity in an electrode contact layer to increase the recombination probability of electrons and holes.
2. Background Art
A schematic stack structure of a general nitride semiconductor light emitting device and a fabrication method thereof will now be described.
FIG. 1 is a sectional view of a general nitride semiconductor light emitting device.
Referring to FIG. 1, a conventional nitride semiconductor light emitting device includes a substrate 101, a buffer layer 103, an n-GaN layer 105, an active layer 107 and a p-GaN layer 109. Herein, the substrate 101 can be exemplified by a sapphire substrate.
A fabrication method of the nitride semiconductor light emitting device will now be described. In order to minimize the occurrence of crystal defects due to differences in the lattice constants and the thermal expansion coefficients of the substrate 101 and the n-GaN layer 105, a GaN-based nitride or an AlN-based nitride having an amorphous phase at a low temperature is formed as the buffer layer 103.
The n-GaN layer 105 doped with silicon at a doping concentration of 1018/cm3 is formed at a high temperature as a first electrode contact layer. Thereafter, the growth temperature is lowered and the active layer 107 is formed. Thereafter, the growth temperature is again elevated and the p-GaN layer 109 doped with magnesium (Mg) and having a thickness range of 0.1-0.5 μm is formed as a second electrode contact layer. The nitride semiconductor light emitting device having the aforementioned stack structure is formed in a p-/n-junction structure which uses the n-GaN layer 105 as the first electrode contact layer and uses the p-GaN layer 109 as the second electrode contact layer.
Also, a second electrode material formed on the second electrode contact layer is limited depending on a doping type of the second electrode contact layer. For example, in order to decrease the contact resistance between the second contact material and the p-GaN layer 109 having a high resistance component and enhance the current spreading, a thin transmissive resistance material of a Ni/Au alloy is used as the second electrode material.
To form the p-GaN layer 109 used as the second electrode contact layer, the p-/n-junction light emitting device using the nitride semiconductor employs a doping source of Cp2Mg or DMZn. In the case of DMZn, since Zn is in ‘deep energy level’ within the p-GaN layer 109 and has a very high activation energy, the hole carrier concentration serving as a carrier when a bias is applied is limited to about 1×1017/cm3. Accordingly, Cp2Mg MO (metal organic) having a low activation energy is used as the doping source.
Also, when the Mg-doped p-GaN layer 109 having a thickness range of 0.1-0.5 μm is grown using a doping source of Cp2Mg at the same flow rate or by sequentially varying the flow rate of Cp2Mg, hydrogen (H) gas separated from the doping source and NH3 carrier gas are combined to form an Mg—H complex, in the p-GaN layer 109, which shows a high resistance insulation characteristic of more than ˜106Ω. Accordingly, in order to emit light during the recombination process of holes and electrons in the active layer 107, an activation process is essentially required to break the bond of Mg—H complex. Since the Mg-doped p-GaN layer 109 has a high resistance, it cannot be used without any change. The activation process is performed through an annealing process at a temperature range of 600° C.-800° C. in an ambient of N2, N2/O2. However, since Mg existing in the p-GaN layer 109 has a low activation efficiency, it has a relatively high resistance value compared with the n-GaN layer 105 used as the first electrode contact layer. In real circumstance, after the activation process, the atomic concentration of Mg in the p-GaN layer 109 is in a range of 1019/cm3-1020/cm3, and the hole carrier concentration contributing to a pure carrier conductivity is in a range of 1017/cm3-1018/cm3, which correspond to a difference of maximum 103 times. It is also reported that the hole mobility is 10 cm2/vsec, which is a very low value. FIG. 2 is a schematic view showing a sectional structure of the conventional Mg-doped p-GaN layer and an Mg profile inside the Mg-doped p-GaN layer after the activation process is performed. Referring to FIG. 2, it can be seen that the Mg atomic concentration and the hole carrier concentration show a difference of maximum 103 times.
Meanwhile, the Mg atomic concentration remaining in the p-GaN layer 109 without a complete activation causes many problems. For example, light emitting from the active layer toward the surface is trapped to lower the optical power, or when a high current is applied, heat is generated due to a relatively high resistance value, so that the life time of the light emitting device is shortened to have a fatal influence on the reliability. Especially, in the case of a large size/high power 1 mm×1 mm light emitting device using a flip chip technique, since a current of 350 mA which is very higher than a conventional current of 20 mA is applied, a junction temperature of more than 100° C. is generated at a junction face, having a fatal influence on the device reliability and causing a limitation to product application in future. The generated much heat is caused by an increase of resistance component due to the Mg atomic concentration remaining in the p-GaN layer 109 used as the second electrode contact layer without being activated as a carrier, and a rough surface property due to the increase of the resistance component.
Also, in the general p-/n- junction light emitting device, the n-GaN layer 105 used as the first electrode contact layer can easily control the hole concentration within 5-6×1018/cm3 within a critical thickness ensuring the crystallinity in proportional to the silicon doping concentration depending on an increase in the flow rate of SiH4 or Si2H6, whilst in the p-GaN layer 109 used as the second electrode contact layer, the hole concentration substantially serving as carriers is limited within a range of 1-9×1017/cm3 although the flow rate of Cp2Mg is increased and Mg atoms of more than maximum ˜1020/cm3 are doped. To this end, the conventional light emitting device is made in a p-/n- junction structure having an asymmetric doping profile.
As aforementioned, the low carrier concentration and high resistance component of the p-GaN layer 109 used as the second electrode contact layer cause the light emitting efficiency to be decreased.
To solve the above problem, a conventional method for increasing the optical power by employing Ni/Au TM (transparent thin metal) having a good transmission and a low contact resistance has been proposed. However, the conventional method badly influences the device reliability when being applied to a large size/high power light emitting device. This problem still remains unsettled in the light emitting devices using the GaN semiconductor.